In various display devices including liquid crystal displays, transparent protective sheets are used on the surfaces of the devices. Regarding these transparent sheets, glass sheets have been used; however, plastic sheets have also come to be used for the purpose of enhancement of machinability, enhancement of safety, improvement of lightweightness, and the like.
In mobile telephones that have been hitherto developed, display screens are intended for displaying characters or images, and operation of the display screens involves pressing of a key button that is separately provided. In recent years, touch panels for operation are increasingly employed in many of various display devices including mobile telephones. In these display devices, signals can be input by directly touching the display screen. Regarding these touch panels for screen operation, resistive film type touch panels have been used in many cases; however, the use of electrostatic capacitance type touch panels is recently increasing, because these touch panels have diverse operability.
An electrostatic capacitance type touch panel is a touch panel of a system by which minute electrical changes caused by finger movement, that is, changes in the electrostatic capacitance, are captured, and thereby positions are detected. Since an electrostatic capacitance type touch panel is capable of detecting the movement of a finger only by having the finger come close to the touch panel without directly touching the sensor, a cover formed from a glass sheet can be attached to the surface of the touch panel. Therefore, it has become possible to provide a display device which has excellent designability, high resistance to scratches, and excellent durability and resistance to environment.
Regarding the transparent sheet that constitutes a display, a glass sheet, particularly a reinforced glass sheet is used in many occasions. However, there is a problem that it is difficult to process a glass sheet by cutting, preventive measures against scattering are also required when the glass sheet has broken, and the cost is expensive.
Accordingly, transparent plastic sheets such as a methacrylic resin sheet or a polycarbonate resin sheet, which are lightweight and have excellent processability compared to glass, are now used for various transparent sheets for display. Under such circumstances, in regard to a sheet for display used in a touch panel system, particularly in an electrostatic capacitance type touch panel, there has been a demand to increase the relative dielectric constant of the transparent plastic sheet in order to increase the response speed when a finger or the like touches the plastic sheet.
As a technique for increasing the relative dielectric constant, a technique of adding inorganic fine particles to a resin is known (Patent Document 1).